Mail Servers

Mail servers come in different forms.

• Mail Transfer Agents (MTA) typically speak SMTP and perhaps LMTP.
• Mail Delivery Agents (MDA) speak IMAP, POP3 usually also supporting SSL/TLS (IMAPS, POP3S).

These mail servers can do some or all of the above.

• Exim
• Postfix
• Qmail
• Sendmail
• Cyrus
• Dovecot
• uw-imap

Mail Plugins

These are add-ons to mail servers that enhance the functionality.

• SpamAssassin - analyze & score message content using bayesian filters
• Amavisd-new - meta-wrapper for spamassassin and a large number of anti-virus programs
• Sieve - a plugin for Dovecot to filter messages just before delivery
• Postgrey - a grey-listing agent for Postfix
• Procmail - a powerful filter/classifier utility

Related

• ClamAV
• Email Blocklists